The present invention relates to an apparatus for depositing a semiconductor film by a vapor phase epitaxy (VPE) process and a method for depositing a semiconductor film using the apparatus. More particularly, this invention relates to an apparatus and method for depositing a semiconductor film while decontaminating a reactor or preventing the film being deposited from being contaminated with reactants adhered to, and then dropped off, the walls of the reactor.
A Group III-V nitride compound semiconductor, represented by the general formula AlxGayIn1-x-yN (where 0xe2x89xa6x+yxe2x89xa61), is effectively applicable as a material for a violet or blue light emitting diode (LED) or a blue light emitting laser diode (LD) for use in high-density optical recording. A nitride compound semiconductor film is normally deposited by a metalorganic vapor phase epitaxy (MOVPE) or hydride vapor phase epitaxy (HVPE) process. Specifically, in an MOVPE process, source gases of Group III and V elements (e.g., a gas of an organic metal like trimethylgallium (TMG) and an ammonia (NH3) gas, respectively) are supplied onto a wafer and allowed to react with each other at about 1000xc2x0 C., thereby growing nitride semiconductor crystals thereon. In an HVPE process on the other hand, source gases of a Group III chloride and a Group V element are supplied onto a wafer and allowed to react with each other at about 1000xc2x0 C., thereby growing nitride semiconductor crystals thereon. In the latter process, the Group III chloride may be obtained by making a Group III metal element such as gallium (Ga) react with hydrogen chloride (HCl), while the Group V element may be ammonium, for example.
A reactor for use in the MOVPE or HVPE process should be made of a material that does not react with H2 or NH3 gas so easily at the elevated temperature of 1000xc2x0 C or more. So in the prior art, part of the reactor to be heated is made of quartz or graphite coated with silicon carbide (SiC), for example.
The known semiconductor film deposition apparatus (i.e., the MOVPE or HVPE reactor), however, has the following two drawbacks.
Firstly, the semiconductor film being deposited on the wafer is likely contaminated with some impurities other than the Group III and V elements. For example, where a nitride compound semiconductor film is deposited using a reactor made of quartz, the heated quartz reacts with a source or carrier gas (e.g., NH3 or H2 gas), thus carrying silicon (Si) and oxygen (O) atoms, i.e., constituent elements of quartz, onto the wafer. As a result, the semiconductor film deposited on the wafer will eventually contain these contaminants at a concentration of about 1013 cmxe2x88x923 to about 1018 cmxe2x88x923.
A nitride compound semiconductor film, which has been deposited using a reactor made of SiC., will also contain contaminants like Si and C atoms at a concentration of about 1013 cmxe2x88x923 to about 1018 cmxe2x88x923. Even though their amounts are usually small, these impurities other than the Group III and V elements likely deteriorate the characteristics of a semiconductor device that will be formed out of the semiconductor film deposited. For example, the operating current of the semiconductor device increases unexpectedly.
Secondly, some flaky reactants likely drop off the inner walls of the reactor onto the semiconductor film being deposited on the wafer. Most of the reactants of the source gases deposit themselves on the wafer, but some of them may be deposited on the surrounding walls of the reactor that have been heated to about 300xc2x0 C. Normally, the nitride compound semiconductor reactants poorly adhere to the walls of the reactor made of quartz or silicon carbide. Accordingly, the reactants, once deposited on the walls, easily drop off the walls during the film deposition process and some flaky ones of the reactants drop off the walls onto the film being deposited on the wafer. If those flaky contaminants dropped onto the film, part of the film could not be formed as originally intended. As a result, a semiconductor device that will be formed out of the film could not operate properly.
It is therefore an object of the present invention to prevent a semiconductor film being deposited by a VPE process from being contaminated with those droppings.
An inventive apparatus is for use to deposit a semiconductor film on a wafer, which is held on a holder inside a reactor, with at least one source gas supplied onto the wafer. To achieve the above object, the apparatus includes a decontamination film made of a semiconductor that contains at least one constituent element of the semiconductor film to be deposited. The decontamination film covers inner walls of the reactor, which are located upstream with respect to the source gas and/or over the holder.
Using the inventive apparatus, the semiconductor film being deposited on the wafer will contain no contaminants including elements other than the constituent elements of the semiconductor film. Accordingly, a semiconductor device, which will be formed out of the semiconductor film deposited on the wafer, will not have its operating characteristics deteriorated.
It should be noted that a similar technique is disclosed in Japanese Laid-Open Publication No. 10-284425, in which a platter, a type of wafer holder, is coated with a nitride semiconductor film to deposit a crystal layer uniformly on a wafer. The coating, however, cannot eliminate the contaminants completely and the technique is much less effective than the technique of the present invention.
In one embodiment of the present invention, the decontamination film preferably also covers parts of the holder that had been exposed inside the reactor before the decontamination film was formed thereon.
In another embodiment of the present invention, the decontamination film may be made of a Group III-V compound semiconductor. Then, the reactants, which will be deposited on the wafer to form the intended semiconductor film thereon, strongly adheres to the decontamination film. Accordingly, no flakes will drop onto the film being deposited on the wafer.
Specifically, the decontamination film preferably covers the inner walls of the reactor and parts of the holder that would have been exposed to the source gas without the decontamination film and that have a temperature of about 300xc2x0 C. or more at the surface thereof when the reactor is heated.
Alternatively, the decontamination film may contain aluminum. This is because where the Group III-V compound semiconductor contains aluminum, the semiconductor film will have a higher decomposition temperature and the decontamination film can adhere to the specified parts more strongly.
More particularly, an aluminum mole fraction of the decontamination film is preferably greater than that of the semiconductor film being deposited on the wafer. Then, the decomposition temperature of the decontamination film is higher than that of the semiconductor film being deposited on the wafer. Accordingly, the decontamination film does not disappear while the semiconductor film is being deposited.
In this particular embodiment, the decontamination film is preferably made of aluminum nitride.
In still another embodiment, the reactor may be made of quartz.
In yet another embodiment, the holder may be made of graphite, and the decontamination film may be formed on an undercoat film that covers the surface of the holder and that contains silicon carbide or boron nitride. In that case, even if the holder is made of graphite, the decontamination film can strongly adhere to the holder, because the decontamination film is formed on an undercoat film of silicon carbide or boron nitride.
A first inventive method for depositing a semiconductor film includes the step of a) introducing source gases, containing aluminum and nitrogen, respectively, into a reactor including a holder therein while heating the reactor before a wafer is loaded into the reactor, thereby forming a decontamination film, containing aluminum and nitrogen, on inner walls of the reactor and on parts of the holder exposed to the source gases. The method further includes the step of b) placing a wafer on part of the decontamination film that has covered the holder and then supplying Group III and V source gases onto the wafer, thereby depositing a Group III-V compound semiconductor film on the wafer. And the method further includes the step of c) unloading the wafer, on which the Group III-V compound semiconductor film has been deposited, from the reactor, and then heating the reactor and the holder up to a temperature that is equal to or higher than a decomposition temperature of the Group III-V compound semiconductor film and equal to or lower than a decomposition temperature of the decontamination film, thereby removing excessive reactants that have deposited themselves on the decontamination film covering the inner walls of the reactor and the holder.
In the first method, a decontamination film is formed inside a reactor before a Group III-V compound semiconductor film is deposited on a wafer. After the film has been deposited thereon, the wafer is unloaded from the reactor and then the reactor and holder are heated to a temperature equal to or higher than a decomposition temperature of the Group III-V compound semiconductor film and equal to or lower than that of the decontamination film. In this manner, excessive reactants, which deposited themselves on the decontamination film covering the inner walls of the reactor and the holder while the semiconductor film was deposited, can be removed. Accordingly, every time a wafer is loaded into the reactor, a desired semiconductor film can be deposited on the wafer uniformly enough.
In one embodiment of the present invention, the step c) may be carried out within a reducing environment. In the reducing environment, a Group III-V compound semiconductor film (e.g., a nitride compound semiconductor film, in particular) has a relatively low decomposition temperature compared to an inert gas environment. Thus, the reactants deposited can be removed at a lower temperature.
Specifically, the reducing environment preferably contains hydrogen gas. Then, the excessive reactants are removable without contaminating the apparatus because hydrogen gas can have very high purity through purification.
A second inventive method for depositing a semiconductor film includes the step of a) introducing source gases, containing aluminum and nitrogen, respectively, into a reactor including a holder therein while heating the reactor before a wafer is loaded into the reactor, thereby forming a decontamination film, containing aluminum and nitrogen, on inner walls of the reactor and on parts of the holder exposed to the source gases. The method further includes the step of b) placing a wafer on part of the decontamination film that has covered the holder and then forming an undercoat semiconductor layer, having substantially the same composition as the decontamination film, on the principal surface of the wafer. The method further includes the step of c) supplying Group III and V source gases onto the undercoat semiconductor layer, thereby depositing a Group III-V compound semiconductor film on the undercoat semiconductor layer.
In the second method, a decontamination film is formed in advance inside a reactor, and then an undercoat semiconductor layer, having substantially the same composition as the decontamination film, is formed on the principal surface of the wafer. Accordingly, every time a wafer is loaded into a reactor, the surface of the decontamination film already existing inside the reactor is covered with reactants having substantially the same composition as the decontamination film. Thus, the decontamination film can have its lifetime extended remarkably.
In one embodiment of the present invention, the second method may further include the step of d) unloading the wafer, on which the Group III-V compound semiconductor film has been deposited, from the reactor, and then heating the reactor and the holder up to a temperature that is equal to or higher than a decomposition temperature of the Group III-V compound semiconductor film and equal to or lower than that of the decontamination film, thereby removing excessive reactants that have deposited themselves on the decontamination film covering the inner walls of the reactor and the holder. Then, excessive reactants, which deposited themselves on the decontamination film covering the inner walls of the reactor and the holder while the semiconductor film was deposited, can be removed. Accordingly, every time a wafer is loaded, a desired semiconductor film can be deposited on the wafer uniformly enough.
In another embodiment of the present invention, the step c) may include depositing the Group III-V compound semiconductor film that has an aluminum mole fraction smaller than that of the decontamination film. Then, the reactants having the lower Al mole fraction are removable from the decontamination film in the step d), thus eliminating flakes almost completely.
In still another embodiment, the step d) may be carried out within a reducing environment. In that case, the reducing environment preferably includes hydrogen gas.